The invention relates to the field of wind turbine generators. Specifically, the invention relates to a controller for permanent magnet, direct current wind turbines.
Wind turbines have gained widespread use for electricity generation in recent years and a growing market is small-scale turbines for battery charging or residential use. Small-scale wind turbines typically utilize a permanent magnet alternator to convert rotational power in the turbine""s rotor into useful electrical power. Permanent magnet alternators have many advantages that cause them to be well suited for use in a wind turbine. Their simplicity, durability, and efficiency are excellent for wind turbine applications.
Permanent magnet alternators, however, also have several weaknesses that must be overcome when designing a wind turbine generator. The first problem is that the alternator tends to lock into a preferred tooth-magnet position and a relatively high wind gust may be needed to initiate rotation of the alternator. Another problem with permanent magnet alternators is that their power output increases linearly with rotational speed whereas, for a wind turbine to maintain optimum aerodynamic efficiency, the alternator""s power should increase with the cube of the rotational speed. Designing a wind turbine to operate at maximum efficiency at a design wind speed with sub-optimum efficiency at all other wind speeds typically gets around this problem. The next problem is that when an alternator is directly coupled to a wind turbine rotor, its output is at a low voltage unless a large number of turns of very fine wire are used in constructing the windings. Using such fine wire results in high electrical resistance and low efficiency.
A permanent magnet alternator typically includes three sets of windings in the stator and the output of the alternator is three phase power with varying voltage and frequency. In order to use the output power for battery charging or other useful purposes, the output is typically rectified to direct current. The rectificiation is most commonly achieved with a diode bridge as shown in FIG. 3. The circuit shown in FIG. 3 will provide a direct current output, but the voltage still varies. A voltage regulating controller is typically utilized for battery charging and other applications.
The prior art uses of permanent magnet alternators do not allow for aerodynamic stall of the wind turbine blades. This is because the speed of the rotor continues to increase as long as the wind speed increases. To achieve power regulation with a permanent alternator wind turbine, it is typical to provide a tail vane that furls the rotor out of the wind, or to design some other power limiting scheme built into the mechanical and aerodynamic design of the wind turbine. However, it would be desirable to have a permanent alternator that could provide increasing torque loads above a certain speed or power level in order to slow the wind turbine""s rotor and induce aerodynamic stall.
Another problem with typical prior art uses of permanent alternators for wind turbines is that the output voltage was relatively low. If a user wishes to invert the output power for use in an alternating current application, it is necessary to first boost the output voltage to a high voltage before putting the power through the inverter. The voltage boost is typically built into the inverter and increases the cost and complexity of the inverter. It would therefore be desirable to have a permanent magnet alternator with high voltage direct current output so that an inverter without voltage boost could be utilized.
The present invention solves the problems of the prior art wind turbines by utilizing a novel power electronic controller. The controller uses a power electronics bridge to provide power control and active rectification. The controller uses boost mode techniques to control the alternator. The boost mode allows optimized performance in low winds and provides for aerodynamic stall in high winds.
The controller consists of a rectifier that utilizes a switching device, such as an FET or an IGBT, on each phase along with at least one diode on each phase. The switching devices short the phases together for a short period of time to allow energy storage within the internal inductance of the alternator. When the switches reopen, the energy stored in the alternator""s inductance is released and the output voltage is temporarily boosted. This technique is commonly employed with external inductors used for energy storage. The present invention utilizes the internal inductance of the alternator, rather than an external inductor, to achieve boost mode.
The controller can be used to regulate the voltage of the alternator""s output so that a separate voltage regulator is not required. If the wind turbine is connected to a battery bank, the controller can monitor the batteries"" voltage and regulate the alternator""s output voltage appropriately to achieve efficient battery charging. If the alternator is used with an inverter in an AC application, then the controller can be used to raise the output voltage of the alternator so that the boost portion of the inverter is not required. This simplifies the design of the inverter and improves the economics of grid-connected wind energy conversion.
One advantage of utilizing boost mode in a wind turbine with a permanent magnet alternator is that it improves the wind turbine""s performance in low winds. The controller monitors the turbine""s rotor speed and, when a sufficient speed is reached to allow generation, the switches on all of the phases are momentarily shorted to cause an inductive voltage spike in the alternator""s windings that causes current to begin flowing. The boost mode also raises the output voltage of the alternator to a level that is useful for battery charging even when the rotor is turning at slow speeds.
The boost mode controller can create an audible acoustic noise at the switching frequency of the FETs or IGBTs. The noise is created by abrupt changes in current in the alternator""s stator windings. When the boost mode controller is activated at low wind speeds to enhance the low wind operating characteristics of the wind turbine, its acoustic noise can stand out above the aerodynamic noise created by the wind turbine. The noise can be particularly noticeable if it is at a constant frequency because the noise is tonal whereas aerodynamic noise is atonal. In order to overcome this issue, the switching frequency of the FETs or IGBTs is constantly varied. Varying the switching frequency causes the noise from the alternator to be atonal and makes it much less noticeable.
At high wind speeds, the boost mode controller can be used to increase the reaction torque in the alternator. This allows the wind turbine""s rotor to be slowed in high winds, thereby inducing aerodynamic stall. In this manner, a permanent magnet alternator can be used in a wind turbine that has variable speed operation and power control by aerodynamic stall.